Our working hypothesis for this proposal is that the modulation of platelet factor XIII-catalyzed reactions by calmodulin lowers the requirement for high concentrations of Ca++ in the platelet cytosol. Calmodulin, purified from platelets and bovine brain, will be assessed for its enhancing role on platelet factor XIII-catalyzed cross-linking of cytoskeletal and membrane components. The platelet proteins which contribute to the polymer formation will be identified. The methods will involve (1) the purification and assay of proteins such as factor XIII, calmodulin, phosphodiesterase, myosin and actin by standard protein chemistry and enzymology procedures. (2) binding experiments using affinity chromatrography techniques. (3) aminoacid analysis following enzymatic degradation of proteins for the determination of Epsilon(Gamma-glutamic)-lysine bonds. (4) immunological procedures following electropheretic transfer of peptides. In view of the central role of Ca++ in multiple platelet functions, an understanding of the role of calmodulin in enhancing the properties that we plan to investigate will prove crucial for intepreting physiological as well as pathological events.